Anomalous Properties of Lys Residues Buried in the Hydrophobic Interior of a Protein Revealed with <sup>15</sup>N-Detect NMR Spectroscopy.

Overview

Ionizable residues buried in hydrophobic environments in proteins are essential for many fundamental biochemical processes. These residues titrate with anomalous pK_a values that are challenging to reproduce with structure-based calculations owing to the conformational reorganization coupled to their ionization. Detailed characterization of this conformational reorganization is of interest; unfortunately, the properties of buried Lys residues are difficult to study experimentally. Here we demonstrate the utility of ¹⁵N NMR spectroscopy to gain insight into the protonation state, state of hydration and conformational dynamics of the Nζ amino group of buried Lys residues. The experiments were applied to five variants of staphylococcal nuclease, with internal Lys residues that titrate with pK_a values ranging from 6.2 to 8.1. Direct detection of buried Lys residues with these NMR spectroscopy methods will enable correlation between thermodynamic and structural data as well as unprecedented examination of how conformational transitions coupled to their ionization affect their pK_a values.